This invention relates to a method and apparatus for displaying a video picture and, more particularly, to such a method and apparatus wherein a video picture is displayed on an m.times.n matrix array of light emitting elements with improved brightness control.
A conventional video picture is displayed on the screen of a cathode ray tube (CRT) by scanning a raster across the face of the CRT. Video picture information is transmitted in the form of periodic vertical intervals, such as field intervals, each vertical interval being constituted by a number of horizontal line intervals of video information. Thus, the displayed raster likewise consists of rows of video information displayed during each vertical interval.
It has been proposed that the typical CRT be replaced by an XY matrix array of light emitting elements for the purpose of displaying a video picture. It is thought that this would simplify the display apparatus, such as a television receiver, by eliminating the usual electron beam, horizontal and vertical beam scanning yokes, vacuum envelope and high voltage devices and generators, all of which are presently used in a CRT or in a CRT driving circuit. The XY array could be formed in a relatively simple panel, constructed as a grid arrangement, and being provided merely with row and column electrodes, respectively. Such a simplification over a conventional CRT is quite advantageous.
One significant problem in video display systems using a flat panel X-Y matrix arrangement heretofore known is in obtaining sufficient brightness in the displayed video picture and, moreover, a satisfactory number of different, perceptible brightness levels. Generally, two different techniques have been used in energizing each light emitting element so as to obtain a desired brightness level. One such technique is known as the pulse amplitude modulation (PAM) technique, wherein a particular light emitting element in a row is energized once during each vertical interval for a pre-established duration. The brightness of the light emitted by that element is a function of the amplitude of the pulse which is supplied thereto, this amplitude being related to the brightness level of the sampled picture element which corresponds to that light emitting element. That is, if there are, for example, m line intervals in each vertical interval, such as m line intervals during each field interval, and if each line interval is sampled n times, then a given light emitting element, such as the light emitting element defined by the i-th row and the j-th column is energized as a function of the j-th sample of the i-th line interval in the video signal. However, since a given light emitting element is energized only once during each vertical interval, it is necessary that a very high voltage be supplied to that light emitting element in order to attain a corresponding high brightness level therefrom. Furthermore, because this PAM technique achieves a brightness level corresponding to the sampled amplitude of a picture element, only a relatively small number of different brightness steps can be provided. This, in turn, limits the overall resolution of the displayed video picture.
Another technique is known as the pulse width modulation (PWM) technique wherein a given light emitting element is energized once during each vertical interval for a duration corresponding to the sampled amplitude of an associated picture element. However, since each row of light emitting elements is conditioned sequentially, the maximum energization duration of any such element is no greater than a line interval. Thus, the maximum number of brightness steps which can be discriminated is limited because of this constraint. Also, since there is an inherent time delay in energizing and de-energizing a light emitting element, the number of different brightness steps which can be provided is further limited.
It has been proposed, in U.S. Pat. No. 4,021,607, issued May 3, 1977, and assigned to the same assignee as the present invention, to combine the PAM and PWM techniques. In this system, a quantized sample picture element amplitude is represented by a pulse of predetermined amplitude and duration. Accordingly, a relatively large number of brightness levels can be displayed. However, here too, since the rows of light emitting elements are energized sequentially on a row-by-row basis, the j-th element in the i-th row can be energized only once during each vertical interval, and the duration and amplitude of the current which flows through the light emitting element, and which determines the brightness of the light emitted thereby, still is limited.